haskell# General approach for combining multiple foldrs over the accumulator type?

Recently I've come up with an idea for a function:

```
gridFoldr :: (a -> Grid -> Grid) -> [a] -> Grid -> Grid
gridFoldr f g xs = foldr f xs g
```

So I can easily combine multiple grid computations using the plain (.) operator:

```
let grid' = gridFoldr gridOp1 args1 . gridFoldr gridOp2 args2 $ myGrid
```

But I have a feeling that I'm missing some general haskell concept for using a similar approach without introducing a new function. Am I right? Is there some standard function / package / or maybe some clever Monad to do the task?

Solution

It looks like you are interested in "grid updates", i.e. in functions `Grid -> Grid`

. This type is isomorphic to `Endo Grid`

, which is a well-known monoid.

So, your function is similar to

```
foldGrid :: (a -> Endo Grid) -> [a] -> Endo Grid
foldGrid f = mconcat . map f
```

or even, exploiting more libraries

```
foldGrid = foldMap
```

Indeed, `foldMap`

is a generalization of your function.

```
foldMap :: (Foldable t, Monoid m) => (a -> m) -> t a -> m
```

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